You can play the game and be successful (winning leagues and cups) without spending anything. Getting in to an Association will help to provide the tokens you'll need, otherwise you can get everything you need from videos and rewards, if you spend wisely.
You don't have to play at or close to 100%. If it suits you better, you can play between 60-80%. The teams you'll be drawn against in the league should be at the same level, and are likely to be a lot of abandoned teams, making winning easier.
If you want more of a challenge, you can buy good young players and train them up to produce a team at 90-110%. That will be more competitive, but it's still possible to be successful at that level without spending real money.